Laminated packaging materials having flexibility have been used for packaging liquid food products for many years. For example, milk has been packaged in cartons made from a laminate composed of paperboard substrate with thermoplastic coatings on both surfaces. The surfaces of the carton are heat-sealed together so as to form a package carton of desired shape.
Some food products, such as orange juice, packaged in such cartons, lose their nutritional values due to the permeation of oxygen through the carton walls. It has therefore been common to include an aluminum foil layer with the laminate material, in order to reduce the permeation of oxygen through the walls and to minimize the degradation of the nutrients such as vitamin C. Although aluminum foil is highly effective as a barrier material, its use in cartons may in some cases raise concern from environmental and recycling points of view and it may be deemed appropriate to replace aluminium foil by other barrier materials. Various attempts have been made to develop practical alternatives to aluminum foil. Such alternatives should have excellent oxygen, gas and aroma barrier properties, and be easily disposable after use.
Another problem in the packaging of liquid food products in cartons arises from the structure of the carton. The carton is fabricated from a carton blank or a carton web (composed of a laminate such as the ones discussed above) that is folded along one or more crease lines for formation into the desired shape. In general, portions of the blank are overlapped for sealing which may be accomplished by the application of suitable adhesive or by heat-sealing the thermoplastic layers together. The creasing of the laminate material as mentioned above imposes stresses to the laminated material. These stresses may cause leakage or, at least weaken the laminate material so that subsequent handling of the carton may lead to leakage.
New oxygen barrier materials have emerged from recent developments in plasma deposition technology for plastics films. The food and pharmaceutical packaging industries have shown tremendous interest in substrate films, usually of thermoplastic polyester, coated with a thin silicon oxide layer. These materials show excellent barrier properties as well as tolerance to the thermomechanical stress encountered during the various converting processes in the manufacture of laminated packaging materials.
U.S. Pat. No. 4,888,199 describes the process of depositing a thin film of a silicon oxide on a surface with the use of plasma under controlled conditions. The plasma is formed in a closed reaction chamber, in which the substrate is positioned. The above-mentioned substrate can be formed from metal, glass or certain plastics. The air is pumped out of the chamber until a high degree of vacuum is achieved.
For example, the organic silicon compound such as hexamethyl disiloxane is introduced into the chamber together with oxygen and helium, so the silicon molecules and oxygen molecules are deposited on the surface of the substrate. The resulting film is described as being a thin film that is very hard, scratch-resistant, optically clear and adheres well to a flexible substrate. The disclosure of the patent is hereby incorporated into this specification by reference.
An improved plasma enhanced chamical vapour deposition (PECVD) method process is described in U.S. Pat. No. 5,224,441, which is also incorporated into this specification. In the process mentioned in the patent, the substrate deposited with the silicon oxide is maintained at a temperature of about 10–35° C., preferably 15–25° C. and the substrate may be formed from polyethylene terephthalate (PET) or polycarbonate resin. In this specification, the thickness of the silicon oxide film when used for food packaging, is about 100 Å (Angstrom)–400 Å and the thickness of the substrate is 1.5 microns–250 microns.
However, during these processes, a major concern is the durability of the barrier layer in that it must not crack or delaminate (detach) from the substrate film. Tendency to cracking is controlled by the cohesion of the oxide material to itself, whereas delamination is controlled by the interfacial adhesion between the oxide layer and the substrate film. Thus there remains a need for a silicon oxide coated substrate that is resistant to cracking and delamination, for providing packages having improved gas barrier and durability properties.